Chronic hypoxia is a central feature of many diseases, including ischemic heart disease, cerebrovascular disease, and chronic obstructive pulmonary disease. Understanding the cellular and physiologic responses to chronic hypoxia will provide the basis for therapies for these widely prevalent diseases. The central transcriptional response to hypoxia is mediated by the Prolyl Hydroxylase Domain (PHD):Hypoxia Inducible Factor (HIF) pathway. In this pathway, PHD (which consists of three isoforms) prolyl hydroxylates the ? subunit of HIF (which consists of three isoforms) and targets the latter for degradation. Under hypoxia, PHD activity is attenuated, allowing for the stabilization of HIF-? and the activation of a broad range of genes involved in hypoxic adaptation, such as ones that promote a shift from aerobic to anaerobic metabolism. One might presume that activation of this pathway would be of direct benefit in these diseases. However, it is clear that chronic activation of this pathway leads to two potentially serious adverse effects, pulmonary hypertension and erythrocytosis. Therapeutic manipulation of this pathway mandates identifying means of tempering these adverse effects. Study of the Tibetan population, who have adapted to high altitudes and chronic hypoxia, offers a unique opportunity to pursue this. Strikingly, this population has avoided the pulmonary hypertension and erythrocytosis that afflict low altitude dwellers who ascend to high altitudes. Hence, if one were able to identify the mechanisms by which this occurs, this would allow approaches that could ameliorate these consequences. A large number of independent genome wide studies of the Tibetan population have recently provided convincing evidence for a genetic basis for this adaptation, and they consistently point to two genes, the PHD2 (also known as EGLN1) and HIF2A (also known as EPAS1) genes. In this application, we will focus on the PHD2 gene. The above referenced studies have identified a series of intronic and exonic single nucleotide polymorphisms (SNPs) that are enriched in the Tibetan population. In the initial R21 phase of the proposed project, we will first identify the functionally important SNP through a series of in vitro assays that will include reporter gene, protein:protein interaction, and cell culture-based assays. In the subsequent R33 phase of the proposed project, we will generate a mouse knockin line to model the Tibetan SNP. We will then examine the capacity of this SNP to ameliorate the pulmonary hypertension and erythrocytosis that is seen in two independent models of chronic hypoxia. In one model, we will expose these mice to hypoxia for three weeks. In the second model, we will cross these mice with a recently generated mouse line bearing a knockin Hif2a mutation that displays highly penetrant erythrocytosis and pulmonary hypertension. We anticipate that the proposed studies will identify a pathway by which the hypoxic response can be engaged while minimizing its most serious adverse effects.